The specific aims of this research are to examine the receptors, ligands, and functions of human decidual lymphocytes with a view to understanding their possible roles in implantation and pregnancy. In particular, we wish: 1. To examine the biochemistry and possible functions of novel proteins overexpressed by human decidual natural killer (NK) cells that may be involved in down-modulation of the immune response, to continue analysis of the transcriptional profiles of these cells and to study their cytoxicity and ability to secrete cytokines. 2. To assess the role of invariant NKT (iNKT) cells in the decidua by studying their interaction with CD1d expressed on several cell types, by examining the relation of their Vbeta repertoire to cytokine secretion, by testing the hypothesis that they function to regulate the immune response at the maternal-fetal interface and by examining their complete transcriptional profiles. 3. To examine the expression in the placenta of putative Class I MHC protein ligands for decidual NK and iNKT cells and of the GM-CSF receptor and to examine the interaction of placental extravillous cytotrophoblasts with NK and iNKT cells and with GM-CSF.